This disclosure is directed to a pipeline pig construction and more particularly to a pig constructed of plastic materials and assembled in such a fashion to prevent accidental disassembly in the pipeline. In recent pipeline construction techniques, the pipe has been improved by incorporating a thin plastic lining on the interior of the pipe. Before that, the interior was exposed metal susceptible of corrosion or perhaps the formation of scales. Such bare metal pipes have been improved significantly by the incorporation of plastic coating materials on the interior. In the older pipelines, metal cleaning devices could be incorporated in the pipeline pig. This was common place, particularly in the form of a steel post supporting wiper cups and the like. With the advent of a thin plastic coating, the apprehension has arisen that such metal components in pipeline pigs may be dangerous, perhaps detrimental to the quality of the coating material in the pipe. For these reasons, it is desirable to avoid placing metal components in pigs. Inevitably, while a cleaning pig may provide hundreds of miles of quality service, nevertheless, they may come apart in a pipeline. Metal used in the construction of the pig may then create a scratch or tear on the plastic coating or lining in the pipe, and thereby expose bare metal. When the metal is exposed, there is the risk of corrosion, rust or scale formation at that location. This may ultimately weaken the wall, and any such tear in the coating defeats the integrity of the coating material.
Attempts have been made in the past as evidenced by the structures set forth in U.S. Pat. Nos. 4,365,379 and 4,413,370. These devices show some effort in providing an all plastic pig mandrel construction. It is submitted that the present apparatus constitutes patentable subject matter over the structures of the two disclosures.
This disclosure sets forth a central mandrel cooperative with a nut for assembly of the pig. An important feature is the incorporation of the assembly nut in conjunction with a fastening pin located at the forward end of the pig. A single mandrel size can be used for a range of pipeline diameters. As an example, a central mandrel of perhaps 4 inches diameter can be used for pipe ranging from about 12 inches in diameter up to perhaps 20 inches in diameter. Such a conversion is simply made by installing larger diameter cups. The cups are replaceable sacrificial members. Multiple cups of identical construction are positioned on the pig body. They are reinforced and held in location by means of a spacer disc which provides backing to each cup. As desired, a spacer disc can be placed on each side of the cup to provide controllable stiffening. Moreover, the spacing between adjacent cups can be controllably fixed by placing a spacer sleeve around the mandrel. In the preferred construction, a single spacer sleeve is devised, and it can be stacked on the mandrel with only one between adjacent cups, or with two installed in the preferred arrangement of the components. The number of spacer sleeves can be varied to thus accommodate variations in the length of the assembled pig constructed on the central mandrel.
One very important feature of this apparatus is the ability to assemble a pig which is less likely to come apart. Where the pig is assembled by means of some type of clamp or nut construction at the rear of the pig body, disassembly is more likely. The present approach thus locates the assembly nut on the mandrel at the front end of the pig. Moreover, all the cups are held onto the mandrel by means of an enlargement having the form of a surrounding peripheral lip. The lip engages the last of the several cups and hence supports the entire stack of cups, discs and spacer sleeves. Thus, the number of cups can be increased from N where N is a whole integer equal to or greater than two and the length of the pig can be increased without limit.
Assembly is thus accomplished at the front end of the pig. To the measure that the pig is fairly large and hence requires substantial torque to install and remove a lock nut on the front end of the pig, the nut is threaded to the pig conveniently by including the protruding lugs on the nut to enable the nut to be hammered for installation and removal. This tends to twist the mandrel. The mandrel is constructed with an open-end socket at the front end to permit a nonround member to be inserted therein in the fashion of an Allen wrench. Of course, this can be increased in size so that a very large nonround opening is constructed to thereby secure the mandrel against rotation when the nut is being placed on the pig or removed from the pig body.
This type construciton enables manufacturing tolerance accumulation to be accommodated. For instance, when several cups (N=two or more) are assembled on a mandrel along with reinforcing discs and various spacer sleeves, the accumulation of tolerance can build up. If for instance the variations in construction are up to about 1/16th of an inch, one can hope that the total tolerance accumulation is substantially zero. There is however the risk that the accumulation will not be zero. With four cups, there are as many as 18 components assembled on the mandrel, and with five cups there are typically 23 parts on the mandrel. This number follows the relationship that the end cup is normally accompanied by two discs and one spacer, that being a total of four components, while intermediate cups typically are accompanied by an aggregate of five components. Suffice it to say, that on the accumulation of 18, 23 or 28 components, all with a nominal manufacturing tolerance in thickness, yields tolerance variation along a mandrel. This construction enables such tolerances to be accumulated and precise construction is therefore not required. That is, the nut that is used to assemble to the mandrel and thereby lock-down all the components can accommodate a great variation in assembly.
An alternate embodiment of the present disclosure utilizes multiple discs as the scrapping element. This contrast with the cups described above, and in the preferred embodiment, the forward or leading disc is made especially hard and is undersized or undergaged in contrast with the nominal ID of the pipeline. Additional discs are included which are made softer and they are also oversized relative to the nominal ID of the pipe.